1. Field of the Invention
The present invention relates to a call setting method for a network system in which a plurality of circuit switching networks is connected to an IP packet network.
2. Description of the Prior Art
With the progress of recent network technology, integration between an audio network and a data network is rapidly developed. As one integrated network, a voice over IP (VoIP) network for realizing voice communication through an IP packet network (e.g., the internet or an intranet) is known. The VoIP network is constituted by, for example, an IP packet network, a plurality of line switching networks (e.g., a telephone network), and a plurality of connection devices (e.g., gateway devices) for connecting the IP packet network and the line switching network to each other. Each line switching network has a line switching system accommodating a single terminal device or a plurality of terminal devices. The line switching system is connected to the IP packet network through a single gateway device or a plurality of gateway devices.
In a conventional technology, when a call which passes through an IP packet network is set between line switching networks, the following operation shown in FIG. 13 is performed. In the example shown in FIG. 13, a line switching network on a call-out side has a line switching system 1b which accommodates a terminal 1a. The line switching system 1b is connected to the IP packet network through a gateway device 11. On the other hand, a line switching network on a call-in side has a line switching system 2b which accommodates a terminal 2a. The line switching system 2b is connected to the IP packet network through gateway devices 21, 22, and 23.
In FIG. 13, it is assumed that the terminal 1a serves as a call-out terminal and that the terminal 2a serves as a call-in terminal. In this case, a call is set between the terminal 1a and the terminal 2a, the terminal 1a perform a call-out operation to input the dial number (destination number) of the terminal 2a to the line switching system 1b as a call-in terminal number. At this time, the line switching system 1b transmits a call setting message including the call-in terminal number to the gateway device 21.
When the gateway device 11 receives the call setting message, the gateway device 11 calculates the IP address of a destination from the call-in terminal number included in the call setting message and transmits an IP packet including the call setting message to the calculated IP address. In this example, as an IP address corresponding to the destination number, the IP address of the gateway device 21 is set. For this reason, the transmitted IP packet is transmitted to the gateway device 21 corresponding to the destination through the IP packet network.
When the gateway device 21 receives the IP packet, the gateway device 21 tries to transmit the call setting message to the line switching system 2b. Here, when all the lines for connecting the gateway device 21 and the line switching system 2b are busy, and when obstacles are generated by the lines, the gateway device 21 gives a connection disable notification (release completion message in FIG. 13) that the gateway device 21 cannot make the line switching system on the call-in side to receive the call setting message the gateway device (gateway device 11) on the call-out side through the IP packet network.
When the gateway device 11 receives the connection disable notification until a predetermined period of time has passed after the gateway device 11 receives the IP packet, the gateway device 11 calculates the IP address of another gateway device (gateway device 22 or 23) connected to the circuit switching system 2b, and transmits an IP packet including the call setting message to the IP address. In the example shown in FIG. 13, the IP packet is transmitted to the gateway device 22.
However, when the gateway device 22 cannot transmits the call setting message to the line switching system 2b, as in the above description, the connection disable notification is given to the gateway device 11. When the gateway device 11 receives the connection disable notification within a predetermined period of time, the gateway device 11 calculates the IP address of another gateway device (gateway device 23) again to transmit an IP packet including the call setting message to the gateway device 23.
When it is determined that the gateway device 23 can transmit the call setting message to the line switching system 2b, the gateway device 23 extracts the call setting message from the IP packet and transmits the call setting message to the line switching system 2b on the basis of the call-in terminal number included in the call setting message. When the line switching system 2b receives the call setting message, the line switching system 2b calls the terminal 2a. Thereafter, several procedures are performed, a call is set between the terminal 1a and the terminal 2a. 
When the gateway devices 21, 22, and 23 can transmit the call setting messages received from the gateway devices on the call-out side to the line switching system on the call-in side, the gateway devices 21, 22, and 23 do not transmit the notifications that the call setting messages can be transmitted to the gateway devices on the call-out side. The gateway devices on the call-out side transmit IP packets and then receive connection disable notifications within a predetermined period of time, the gateway devices recognize that the call setting messages are transmitted from the gateway devices on the call-in side to the line switching system on the call-in side, and does not perform the process of calculating the IP address of another call-in side gateway devices.
In the conventional technology, the gateway device on the call-out side must transmit IP packet including call setting message to a gateway device having an IP address corresponding to a call-in terminal number, first. In this manner, in the conventional technology, a gateway device on the call-out side, when there are a plurality of gateway devices which can become a gateway device on the call-in side, cannot transmit the IP packet including the call setting message to an optimum gateway device selected from the plurality of gateway devices.
When the gateway device on the call-out side wait for response from the gateway device corresponding to a destination of the IP packet and receive connection disable notification from the destination gateway device, the gateway device retransmit the IP packet including the call setting message to another gateway device. Such a retransmitting process is repeated every gateway device until a gateway device which can transmit the call setting message to the line switching system on the call-in side is found or until there is no gateway device to which the IP packet must be transmitted.
Therefore, when the gateway device on the call-in side cannot transmit a call setting message to the line switching system on the call-in side, the gateway devices on the call-out side repeatedly transmit the same IP packets to another gateway device. For this reason, a load acting on the IP packet network increases, and messages and data transmitted through the IP packet network may be delayed.